1. Field of the Invention
The present invention relates to activated carbon and methods for preparing same. Particularly, this invention relates to the preparation of gasoline adsorptive activated carbons and their use in emission control canisters for gasoline powered vehicles. More particularly, this invention relates to activated carbon derived from lignocellulosic material prepared by chemical activation, agglomeration, and shaping of the agglomerated carbon.
2. Description of the Prior Art
Activated carbon is a microcrystalline, nongraphitic form of carbon which has been processed to increase internal porosity. Activated carbons are characterized by a large specific surface area typically in the range of 500-2500 m.sup.2 /g, which permits its industrial use in the purification of liquids and gases by the adsorption of gases and vapors from gases and of dissolved or dispersed substances from liquids. Commercial grades of activated carbon are designated as either gas-phase or liquid-phase adsorbents. Liquid-phase carbons generally may be powdered, granular, or shaped; gas-phase, vapor-adsorbent carbons are hard granules or hard, relatively dust-free shaped pellets. The present invention relates to shaped (pelleted), gas-phase, vapor-adsorbent active carbons.
Generally, the larger the surface area of the activated carbon, the greater its adsorption capacity. The available surface area of activated carbon is dependent on its pore volume. Since the surface area per unit volume decreases as individual pore size increases, large surface area generally is maximized by maximizing the number of pores of very small dimensions and/or minimizing the number of pores of very large dimensions. Pore sizes are defined herein as micropores (pore width&lt;1.8 nm), mesopores (pore width=1.8-50 nm), and macropores (pore width&gt;50 nm). Mesopores may be further divided between small mesopores (pore width=1.8-5 nm) and large mesopores (pore width=&lt;5-50 nm).
The liquid adsorptive capacity of the activated carbon relies primarily on large mesopores and macropores. As noted above, high macropore content normally is detrimental to the activated carbon's density characteristics, particularly if the activated carbon is derived from a lignocellulosic material. Microporosity, which may contribute to density, is detrimental to the liquid adsorbent effectiveness of the activated carbon, on a carbon volume basis.
The vapor adsorptive capacity of the activated carbon, on the other hand, relies primarily on micropores and small mesopores; whereas, the macropores reduce the density and can be detrimental to the vapor adsorbent effectiveness of the activated carbon, on a carbon volume basis. The adsorption capacity and rate of adsorption depend to a large extent upon the internal surface area and pore size distribution. Conventional chemically activated lignocellulose-based carbons generally exhibit macroporosity (macropore volume) of greater than 20% of the carbon particle total volume. Gas-phase activated carbon macroporosity of less than 20% of the carbon particle volume would be desirable. Likewise, a high percentage of mesoporosity (i.e., above 50% of total particle volume), particularly small mesoporosity, is desirable.
Due to environmental concerns and regulatory mandates, one of the largest single applications for gas-phase carbon is in gasoline vapor emission control canisters on automobiles. Evaporative emissions vented from both fuel tank and carburetor are captured by activated carbon.
Fuel vapors, vented when the fuel tank or carburetor is heated, are captured in canisters generally containing from 0.5 to 2 liters of activated carbon. Regeneration of the carbon is accomplished by using intake manifold vacuum to draw air through the canister. The air carries desorbed vapor into the engine where it is burned during normal operation. An evaporative emission control carbon should have suitable hardness, a high vapor working capacity, and a high saturation capacity. The working capacity of a carbon for gasoline vapor is determined by the adsorption-desorption capacity differential, by the volume of purge air which flows through the carbon canister, and by the extent to which irreversibly adsorbed, high molecular weight gasoline components accumulate on the carbon.
Wood-based carbons are relatively soft as compared to coal-based carbons. The ability to prepare an activated carbon of a higher density, higher hardness, and smaller median pore size from a material of a lower density, lower hardness, and large median pore size (lignocellulosic material) is taught in U.S. Pat. No. 4,677,086, which disclosure is incorporated herein by reference. An active granular wood-based carbon is ground to a fine powder, mixed with water and a bentonite clay binder, extruded to form cylindrical pellets, oven dried, and heat treated at 1,000.degree. F. The uniform pellet form provides consistent particle size and a regular pellet shape which minimizes pressure drop in gas phase applications. Thus, the terms "high density" and "high activity" are used herein in a relative sense as the invention is limited to processing lignocellulosic material.
Also, U.S. Pat. No. 5,039,651 (which disclosure is also incorporated herein by reference) teaches densification of activated carbon product from cellulose materials including coconut shells, wood chips, and sawdust by pressing after initially heating to a relatively low temperature, followed by extrusion and calcination.
U.S. Pat. No. 5,206,207 (incorporated herein by reference) discloses activated carbons of high activity and relatively high density suitable for solvent and vapor capture and recovery prepared by chemically activating carbonaceous material fragments (i.e., "discrete particles"), heat plasticizing the particles to begin transition to thermoset, densifying the particles to "high density" by mechanical shaping (in a spheronizer), further heating the shaped particles to thermoset, and still further heating the thermoset shaped particles to 425.degree.-650.degree. C. Unfortunately, the spheronizing equipment limitations related to such process restrict capacity to below commercial production levels. The mechanical shaping in the plasticized state of the chemically activated carbon effectively "shifts" the particle pore dispersion by increasing the number of small mesopores and micropores at the expense of (i.e., by reducing the number of) macropores. This provides even higher activity (by increasing surface area in the desirable pore size range) and higher density (see Table I, below).
A more commercially feasible process of making activated carbons of high activity and relatively high density suitable for solvent and vapor capture and recovery is disclosed in U.S. Pat. No. 5,250,491 (incorporated herein by reference) which provides a chemical activation and agglomeration process for producing high activity gas-phase activated carbons without sacrificing improvements in density. U.S. Pat. No. 5,324,703, issued Jun. 28, 1994 from parent application Ser. No. 08/095,755 represents an improvement over the U.S. Pat. No. 5,250,491 process and product by including a pelleting (by extrusion) step before final heat activation to result in a shaped high activity and high density active carbon of improved hardness in the absence of employing a binder during the extrusion step. Upon scale-up of this technology, however, it was discovered that the temperature of the cooking process could not be controlled as easily as under laboratory conditions and, absent control of the temperature of the feed material to the pin mixer up to a temperature of below about 185.degree. C., the extruded, finally activated product exhibited low hardness and reduced physical integrity in a solvent or vapor environment. Therefore, the object of the present invention is to produce a uniformly shaped high activity, high density active carbon suitable for solvent and vapor capture and recovery exhibiting high hardness and enhanced physical integrity using a pin mixer without the observed feed material temperature limitations.